Flutter sensing and control system for a gas turbine engine

ABSTRACT

A method of operation for a gas turbine engine according to an exemplary aspect of the present disclosure includes, among other things, reducing a rotational speed of a fan relative to a shaft through a gear train, driving the shaft with a first turbine, driving a compressor with a second turbine, communicating airflow from the fan through a bypass passage defined by a nacelle, the nacelle extending along an engine axis and surrounding the fan, and having a bypass ratio of greater than 10, discharging the airflow through a variable area fan nozzle defining a discharge airflow area, detecting an airfoil flutter condition associated with adjacent airfoils of the fan, and moving the variable area fan nozzle to vary the discharge airflow area and mitigate the airfoil flutter condition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/725,748, filed 5 Oct. 2017, which is a continuation of U.S. patentapplication Ser. No. 13/340,747, filed 30 Dec. 2011, which is acontinuation of U.S. patent application Ser. No. 11/682,015, which wasfiled on 5 Mar. 2007 and is incorporated herein by reference.

BACKGROUND

This invention generally relates to a gas turbine engine, and moreparticularly to a flutter sensing system for a gas turbine engine.

Gas turbine engines typically include a compressor section, a combustorsection and a turbine section. Air is pressurized in the compressorsection and is mixed with fuel and burned in the combustor section toadd energy to expand the air and accelerate the airflow into the turbinesection. The hot combustion gases that exit the combustor section flowdownstream through the turbine section, which extracts kinetic energyfrom the expanding gases and converts the energy into shaft horsepowerto drive the compressor section.

In a turbofan gas turbine engine, for example, a fan section is includedupstream of the compressor section. Combustion gases are discharged fromthe gas turbine engine through a core exhaust nozzle and fan air isdischarged through an annular fan exhaust nozzle defined at leastpartially by a nacelle surrounding the core engine. A majority ofpropulsion thrust is provided by the pressurized fan air which isdischarged through the fan exhaust nozzle, while the remaining thrust isprovided from combustion gases discharged through the core exhaustnozzle.

A fan section, the compressor section and the turbine section mayinclude multiple airfoils disposed circumferentially about an enginelongitudinal centerline axis. At certain aircraft operating conditions,these airfoils may be subjected to flutter, or self-inducedoscillations. The flutter conditions are caused by the interactionbetween adjacent airfoils. During flutter, aerodynamic forces couplewith each airfoil's elastic and inertial forces, which may increase thekinetic energy of each airfoil and produce negative damping. Thenegative damping is enhanced where adjacent airfoils vibrate in unison.Disadvantageously, the airfoil oscillations caused by flutter may becomeso severe that fracture or failure of the airfoils is possible.

Methods are known for mitigating the negative effects of flutter. Forexample, many gas turbine engine systems include high pressurecompressors having variable vane rows (i.e., vanes that are rotatableabout a perpendicular axis relative to a longitudinal centerline axis ofthe gas turbine engine). The variable vane rows have been usedeffectively to schedule the engine around flutter conditions bycontrolling the angle of incidence of the airfoils relative to adirection of flowing airflow. Also, bleed or valve systems are knownwhich bleed airflow downstream from the airfoils to throttle airflow andmitigate flutter. Additionally, airfoil designs are known which tailor aleading edge of each airfoil to obtain improved local airfoil incidenceand adjacent airfoils having different natural frequencies. Finally,having inconsistent airfoil spacing in a forward stage varies theintermittent air pulses communicated to a following airfoil stage, thusreducing natural frequency excitation. Disadvantageously, all of thesemethods result in system compromises, small to moderate performancelosses and may be expensive to incorporate into existing gas turbineengine systems.

Accordingly, it is desirable to provide a gas turbine engine having aclosed-loop flutter sensing system which achieves reduced flutteroperation and minimizes performance losses of the gas turbine engine.

SUMMARY

A gas turbine engine assembly according to an exemplary embodiment ofthe present disclosure includes, among other things, a nacelle, a coreengine casing within the nacelle, a low pressure turbine having apressure ratio that is greater than five, and a bypass passageestablished between the nacelle and the core engine casing. About 80% ormore of airflow entering the engine is moved through the bypass passage.

In a further non-limiting embodiment of the foregoing gas turbine engineembodiment, about 80% of the airflow entering the engine is movedthrough the bypass passage.

In a further non-limiting embodiment of either of the foregoing gasturbine engine embodiments, the gas turbine engine includes a fan and agear train, the gear train reduces the rotational speed of the fanrelative to a shaft of the gas turbine engine. The shaft is rotatablycoupled to a low pressure compressor of the engine.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, the gear train is a planetary gear train.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, a variable area fan nozzle controls a dischargeairflow area of the bypass passage.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, the discharge airflow area extends between thevariable area fan nozzle and the core engine casing.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, a controller is operable to move the variable areafan nozzle to change the discharge airflow area associated with thevariable area fan nozzle in response to an airfoil flutter condition.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, the controller influences the discharge airflow areaby moving the variable area fan nozzle between a first position having afirst discharge airflow area and a second position having a seconddischarge airflow area greater than the first discharge airflow area inresponse to the airfoil flutter condition.

A gas turbine engine according to another exemplary embodiment of thepresent disclosure includes, among other things, a nacelle, a coreengine casing within the nacelle, a low pressure turbine having apressure ratio that is greater than five, and a bypass passageestablished between the nacelle and the core engine casing. A ratio ofan amount of airflow communicated through the bypass passage to anamount of airflow communicated through the core engine is greater than10.

In a further non-limiting embodiment of the foregoing gas turbine engineembodiment, the gas turbine engine includes a fan and a gear train. Thegear train reduces the rotational speed of the fan relative to a shaftof the gas turbine engine. The shaft is rotatably coupled to a lowpressure compressor of the engine.

In a further non-limiting embodiment of either of the foregoing gasturbine engine embodiments, the gear train is a planetary gear train.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, a variable area fan nozzle that controls a dischargeairflow area of the bypass passage.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, the discharge airflow area extends between thevariable area fan nozzle and a core engine casing.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, a controller is operable to move the variable areafan nozzle to change the discharge airflow area associated with thevariable area fan nozzle in response to the airfoil flutter condition.

In a further non-limiting embodiment of any of the foregoing gas turbineengine embodiments, the controller influences the discharge airflow areaby moving the variable area fan nozzle between a first position having afirst discharge airflow area and a second position having a seconddischarge airflow area greater than the first discharge airflow area inresponse to detection of the airfoil flutter condition.

The various features and advantages of this invention will becomeapparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description arebriefly described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a general partial cut-away view of a gas turbineengine;

FIG. 2 is a perspective view of a section of a variable area fan nozzle(VAFN);

FIG. 3 is a schematic view of an example gas turbine engine having avariable area fan nozzle (VAFN); and

FIG. 4 illustrates a partial cut-away view of a fan section of the gasturbine engine.

DETAILED DESCRIPTION

FIG. 1 illustrates a gas turbine engine 10 which suspends from a pylon11 and may include (in serial flow communication) a fan section 12, alow pressure compressor 14, a high pressure compressor 16, a combustor18, a high pressure turbine 20 and a low pressure turbine 22. Duringoperation, air is pulled into the gas turbine engine 10 by the fansection 12, is pressurized by the compressors 14, 16, and is mixed withfuel and burned in the combustor 18. Hot combustion gases generatedwithin the combustor 18 flow through the high and low pressure turbines20, 22, which extract energy from the hot combustion gases.

In a two spool design, the high pressure turbine 20 utilizes theextracted energy from the hot combustion gases to power the highpressure compressor 16 through a high speed shaft 19, and a low pressureturbine 22 utilizes the energy extracted from the hot combustion gasesto power the low pressure compressor 14 and the fan section 12 through alow speed shaft 21. However, the invention is not limited to the twospool gas turbine architecture described and may be used with otherarchitectures such as a single spool axial design, a three spool axialdesign and other architectures. That is, the present invention isapplicable to any gas turbine engine, and to any application.

The example gas turbine engine 10 is in the form of a high bypass ratioturbofan engine mounted within a nacelle 26, in which a significantamount of the air pressurized by the fan section 12 bypasses the coreengine for the generation of propulsion thrust. The nacelle 26 partiallysurrounds a fan casing 28 and an engine casing 31. The exampleillustrated in FIG. 1 depicts a high bypass flow arrangement in whichapproximately 80% of the airflow entering the fan section 12 may bypassthe core engine via a fan bypass passage 30 which extends between thenacelle 26 and the core engine casing 31 for receiving and communicatinga discharge airflow F1. The high bypass flow arrangement provides asignificant amount of thrust for powering an aircraft.

In one example, the bypass ratio (i.e., the ratio between the amount ofairflow communicated through the fan bypass passage 30 relative to theamount of airflow communicated through the core engine itself) isgreater than ten, and the fan section 12 diameter is substantiallylarger than the diameter of the low pressure compressor 14. The lowpressure turbine 22 has a pressure ratio that is greater than five, inone example. The engine 10 may include a gear train 23 which reduces thespeed of the rotating fan section 12. The gear train 23 can be any knowngear system, such as a planetary gear system with orbiting planet gears,a planetary system with non-orbiting planet gears, or other type of gearsystem. In the disclosed example, the gear train 23 has a constant gearratio. It should be understood, however, that the above parameters areonly exemplary of a contemplated geared turbofan engine. That is, theinvention is applicable to a traditional turbofan engine as well asother engine architectures.

The discharge airflow F1 is communicated within the fan bypass passage30 and is discharged from the engine 10 through a variable area fannozzle (VAFN) 40 defined radially between the nacelle 26 and the coreengine casing 31. Core exhaust gases C are discharged from the coreengine through a core exhaust nozzle 32 defined between the core enginecasing 31 and a center plug 34 defined coaxially therein around alongitudinal centerline axis A of the gas turbine engine 10.

In one example, the VAFN 40 concentrically surrounds the core enginecasing 31 near an aftmost segment 29 of the nacelle 26. However, theVAFN 40 may be positioned at other locations of the engine 10. Adischarge airflow area 36 is associated with the VAFN 40 and extendsbetween the VAFN 40 and the core engine casing 31 for axiallydischarging the fan discharge airflow F1.

FIG. 2 illustrates the components of the VAFN 40. This structure isexemplary only, and, as other embodiments would similarly vary thedischarge airflow area 36, will only be briefly discussed herein. TheVAFN 40 generally includes a synchronizing ring 41, a static ring 43 andat least one flap assembly 45. Other VAFN actuation mechanisms may beused. The flap assembly 45 is pivotally mounted to the static ring 43 atmultiple hinges 47 and linked to the synchronizing ring 41 through alinkage 49. An actuator assembly 51 selectively rotates thesynchronizing ring 41 relative to the static ring 43 to adjust the flapassembly 45 through the linkage 49. The radial movement of thesynchronizing ring 41 is converted to tangential movement of the flapassembly 45 to vary the discharge airflow area 36 of the VAFN 40, as isfurther discussed below.

FIG. 3 illustrates a flutter sensing system 50 of the gas turbine engine10. The discharge airflow area 36 may be influenced during certainflight conditions, such as flutter conditions, by opening or closing theVAFN 40. Flutter conditions represent self-induced oscillations. Flutterconditions are caused by unsteady aerodynamic conditions such as theinteraction between adjacent airfoils. During flutter, aerodynamicforces couple with each airfoil's elastic and inertial forces, which mayincrease the kinetic energy of each airfoil and produce negativedamping. The negative damping is enhanced where adjacent airfoils beginto vibrate together.

In one example, the VAFN 40 is moveable between a first position X and asecond position X′ (represented by phantom lines). A discharge airflowarea 37 of the second position X′ is greater than the discharge airflowarea 36 of the first position X.

The VAFN 40 is selectively moved to the second position X′ to controlthe air pressure of the discharge airflow F1 within the fan bypasspassage 30. For example, closing the VAFN 40 (i.e., moving the VAFN tothe first position X) reduces the discharge airflow area which restrictsthe fan airflow F1 and produces a pressure build up (i.e., an increasein air pressure) within the fan bypass passage 30. Opening the VAFN 40to the second position X′ increases the discharge airflow area, allowingadditional fan airflow, which reduces the pressure build up (i.e., adecrease in air pressure) within the fan bypass passage 30. That is,opening the VAFN 40 creates additional thrust power for the gas turbineengine 10.

The flap assemblies 45 (See FIG. 2) of the VAFN 40 are moved from thefirst position X to the second position X′ in response to detecting aflutter condition of the gas turbine engine 10, in one example. Inanother example, the VAFN 40 is moved in response to detecting across-wind condition. However, it should be understood that the VAFN 40may additionally be actuated in response to other operability conditionssuch as take-off or ground operations.

The flutter sensing system 50 is a closed-loop system and includes asensor 52 and a controller 54. The sensor 52 actively and selectivelydetects the flutter condition and communicates with the controller 54 tomove the VAFN 40 between the first condition X and the second positionX′ or any intermediate position via the actuator assemblies 51. Ofcourse, this view is highly schematic. In one example, the sensor 52 isa time of arrival type sensor. A time of arrival sensor times thepassage (or arrival time) of an airfoil as the airfoil passes a fixed,case-mounted sensor as the airfoil rotates about the engine longitudinalcenterline axis A. In the example shown in FIG. 3, the arrival time ofthe fan section 12 airfoils 60 are timed by the sensor 52. Of course,other airfoils may similarly be timed. The controller 54 is programmedto differentiate between which airfoil arrival times correlate to aflutter condition and which airfoil arrival times correlate tonon-flutter conditions.

It should be understood that the sensor 52 and the controller 54 areprogrammable to detect flutter conditions or other conditions. A personof ordinary skill in the art having the benefit of the teachings hereinwould be able to select an appropriate sensor 52 and program thecontroller 54 with the appropriate logic to communicate with the sensor52 and the actuator assembly 51 to move the VAFN 40 between the firstposition X and the second position X′ or any intermediate position inresponse to a flutter condition or any other condition.

The VAFN 40 is returned to the first position X from the second positionX′, which is otherwise indicated when the flutter conditions subside. Inone example, the sensor 52 communicates a signal to the controller 54where the flutter conditions are no longer detected by the sensor 52.Therefore, the efficiency of the gas turbine engine 10 is improvedduring both flutter and non-flutter conditions. Also, airfoil damage dueto continued operation in a flutter condition is reduced.

FIG. 4 illustrates an example mounting location for the sensor 52 of theflutter sensing system 50. In one example, the sensor 52 is mounted tothe fan casing 28 which surrounds the fan section 12. In anotherexample, the sensor 52 is mounted directly adjacent to a blade tip areaT of the fan section 12. The blade tip area T of the fan section 12 isthe area of the fan casing 28 which is directly adjacent to the tips 62of each airfoil 60 (only one shown in FIG. 4) of the fan section 12 asthe airfoils 60 are rotated about the engine centerline axis A. In yetanother example, multiple sensors 52 are circumferentially disposedabout the core engine casing 31 adjacent to the blade tip area T of eachairfoil 60. The sensor 52 may also be mounted adjacent to the blade tiparea of the airfoils of the compressor sections 14, 16 or the turbinesections 20, 22.

The foregoing description shall be interpreted as illustrative and notin any limiting sense. A worker of ordinary skill in the art wouldrecognize that certain modifications would come within the scope of thisinvention. For that reason, the following claims should be studied todetermine the true scope and content of this invention.

What is claimed is:
 1. A method of operation for a gas turbine engine,comprising: reducing a rotational speed of a fan relative to a shaftthrough a gear train; driving the shaft with a first turbine; driving acompressor with a second turbine; communicating airflow from the fanthrough a bypass passage defined by a nacelle, the nacelle extendingalong an engine axis and surrounding the fan, and having a bypass ratioof greater than 10; discharging the airflow through a variable area fannozzle defining a discharge airflow area; detecting an airfoil fluttercondition associated with adjacent airfoils of the fan; and moving thevariable area fan nozzle to vary the discharge airflow area and mitigatethe airfoil flutter condition; providing a flutter sensing systemincluding at least one sensor that actively and selectively detects theairfoil flutter condition in operation, and communicates with acontroller programmed to move the variable area fan nozzle to vary thedischarge airflow area; and wherein the step of moving the variable areafan nozzle includes moving the variable area fan nozzle between a firstposition having a first discharge airflow urea and a second positionhaving a second discharge airflow area greater than the first dischargeairflow area, and further composing returning the variable urea fannozzle to the first position once the flutter condition is no longerdetected by the at least one sensor.
 2. The method as recited in claim1, further comprising differentiating between airfoil arrival times thatcorrelate to the airfoil flutter condition and airfoil arrival timesthat correlate to a non-flutter condition.
 3. The method as recited inclaim 1, wherein the flutter sensing system is a closed-loop system. 4.The method as recited in claim 1, wherein the at least one sensor ismounted to an engine structure adjacent to a blade tip area.
 5. Themethod as recited in claim 1, wherein the at least one sensor is a timeof arrival type sensor.
 6. The method as recited in claim 1, wherein thevariable area fan nozzle concentrically surrounds a core engine casingnear an aftmost segment of the nacelle.
 7. The method as recited inclaim 6, wherein the variable area fan nozzle is defined radiallybetween the nacelle and the core engine casing, and further comprisingdischarging core exhaust gasses from a core engine through a coreexhaust nozzle defined between the core engine casing and a center plug,the core engine comprising the compressor, the first turbine and thesecond turbine.
 8. The method as recited in claim 7, wherein the atleast one sensor includes a first sensor and a second sensor, the secondsensor disposed about the core engine casing.
 9. The method as recitedin claim 8, wherein the gear train is a planetary gear system withorbiting planet gears.
 10. The method as recited in claim 1, wherein theat least one sensor is a time of arrival type sensor.
 11. The method asrecited in claim 10, wherein the first position is radially outward fromthe second position with respect to the engine axis.
 12. The method asrecited in claim 1, wherein the variable area fan nozzle includes atleast a synchronizing ring, a static ring and at least one flapassembly.
 13. The method as recited in claim 12, wherein the at leastone flap assembly is pivotally mounted to the static ring and linked tothe synchronizing ring.
 14. The method as recited in claim 13, whereinthe step of moving the variable area fan nozzle to vary the dischargeairflow area includes rotating the synchronization ring relative to thestatic ring, wherein radial movement of the synchronizing ring isconverted to tangential movement of the at least one flap.
 15. Themethod as recited in claim 13, wherein the gear system train is aplanetary gear system with orbiting planet gears.
 16. The method asrecited in claim 15, further comprising driving another compressor withthe first turbine, wherein the first turbine is a 3-stage turbine, andthe second turbine is a 2-stage turbine.
 17. The method as recited inclaim 1, further comprising driving another compressor with the firstturbine, wherein the first turbine is a 3-stage turbine, and the secondturbine is a 2-stage turbine.
 18. The method as recited in claim 17,wherein the gear train is a planetary gear system with orbiting planetgears.
 19. The method as recited in claim 18, wherein the first positionis radially outward from the second position with respect to the engineaxis.
 20. A method of operation for a gas turbine engine, comprising:reducing a rotational speed of a fan relative to a shaft through a geartrain, the fan including a plurality of airfoils; driving the shaft witha first turbine; driving a first compressor with the shaft; driving asecond compressor with a second turbine; communicating airflow from thefan through a bypass passage defined by a nacelle, the nacelle extendingalong an engine axis and surrounding the fan, and defining a bypassratio of greater than 10; discharging the airflow through a variablearea fan nozzle defining a discharge airflow area; detecting the airfoilflutter condition with at least one sensor of a flutter sensing system,the flutter sensing system being a closed-loop sensor and including acontroller in communication with the at least one sensor and programmedto move the variable area fan nozzle; moving the variable area fannozzle in response to the controller to vary the discharge airflow areaand mitigate the airfoil flutter condition; wherein the at least onesensor is mounted adjacent to a blade tip area; and wherein the variablearea fan nozzle concentrically surrounds a core engine casing near anaftmost segment of the nacelle, and the variable area fan nozzle isdefined radially between the nacelle and the core engine casing; anddischarging core exhaust gasses from a core engine through a coreexhaust nozzle defined between the core engine casing and a center plug,the core engine comprising the first compressor, the second compressor,the first turbine and the second turbine.
 21. The method as recited inclaim 20, wherein the at least one sensor is a time of arrival typesensor.
 22. The method as recited in claim 21, wherein the blade tiparea is located in the first compressor, the second compressor, thefirst turbine or the second turbine.
 23. The method as recited in claim21, wherein the blade tip area is located adjacent the fan.
 24. Themethod as recited in claim 23, further comprising differentiating withthe controller between the airfoil flutter condition and a non-fluttercondition.
 25. The method as recited in claim 24, further comprisingreturning the variable area fan nozzle to a first position from a secondposition once the flutter condition is no longer detected by the atleast one sensor.
 26. The method as recited in claim 25, wherein the gasturbine engine is a two-spool engine, the second turbine is a two-stageturbine, the first turbine is a three-stage turbine, and the gear trainhas a constant gear ratio.
 27. The method as recited in claim 26,wherein the gear tram is a planetary gear system with orbiting planetgears.
 28. The method as recited in claim 27, wherein the variable areafan nozzle includes at least a synchronizing ring, a static ring and atleast one flap assembly.
 29. The method as recited in claim 28, wherein:the at least one flap assembly is pivotally mounted to the static ringand linked to the synchronizing ring; and the step of moving thevariable area fan nozzle in response to the controller to vary thedischarge airflow area and mitigate the airfoil flutter conditionincludes rotating the synchronization ring relative to the static ring,wherein radial movement of the synchronizing ring is converted totangential movement of the at least one flap.